IIRC my husband built mine two feet wide (deep) and there are two five foot long sections end to end so 10 feet total. We have two roosts over the board with one slightly higher than the other, so I ended up with 20 feet of roost space available. We framed the edges so it is a "box" and I keep it lined with PDZ. If you center your roost over the board you have available you *should* be okay - if the occasional poop makes it over one side or the other it's still going to catch the majority of it and that's the idea.

Are your 2 roosts equally spaced over the poop board? Example: 1st roost 8 inches from wall, next roost 8" from 1st roost, which is 8" from the edge of the board. Do you have a lot of spillage over the side?

My poop boards are only about 12" below my roosts. I use dried leaves to catch the droppings and it works amazing. I find the leaves keep the odor down and break down better for compost than pine shavings. The sand method for me would be way too much work to sift the poop out everyday as I have a large number of chickens.